The present invention relates to a high-brightness compressing circuit used in a video signal appliance such as namely a video camera or an imaging device. The present invention relates particularly to a high-brightness compressing circuit which can improve a color-reproduction ability when any of three, red (R), green (G), and blue (B), video signals exceeds a knee-point which represents a level for starting the high-brightness compression.
A three-panel video camera includes three solid state imaging devices such as a charge coupled device (CCD) for three primary colors, red (R), green (G), and blue (B). Their dynamic range is commonly linear up to 600% of an equivalent amplitude to the maximum brightness of video signals. However, as the maximum level of an output signal of the video camera is limited to about 110%, any excessive level can hardly be transmitted to the succeeding devices.
When the video signal is simply clipped at its maximum level, its level exceeding the maximum level fails to be reproduced in gradation. In other words, a higher brightness region of a captured object is saturated. For reproduction of optimum gradation of the higher brightness region, the video camera usually includes a high-brightness compressing circuit known as a knee-compressing circuit.
As shown in FIG. 10, an input/output characteristic of a conventional high-brightness compressing circuit where the video signal which is higher than a particular level is compressed so that its characteristic curve gradient may be small. The particular level is called a knee-point kp, which is smaller than the maximum output level. In general, the knee-point is common to each color video signal. The gradient is called a knee-slope and common for each color video signal. Such a conventional high-brightness compressing circuit allows the video signal exceeding the knee-point to be compressed so that its gradation may be reproduced, and its higher brightness region is prevented from being saturated because the region does not reach the maximum level.
In the conventional high-brightness compressing circuit, as the knee-compression is performed to the three, R, G, and B, color video signals separately, a ratio among the color signals may be altered before and after the knee-compression. More specifically, when the three color signals are produced from a color object, the knee-compression is first performed to their largest signal of the three. At a higher brightness region of the object, the all three, R, G, and B, color signals are compressed, a difference between any two of the three color signals becomes small, and a reproduced color hence changes to almost white.
The above procedure for decreasing the color saturation of the higher brightness region of an image may produce a natural visual effect. However, as the knee-compression starts with the largest of the three color signals, it may change the hue during decreasing the color saturation. For example, a skin color may be turned to yellowish and then to white as its brightness increases.
Some techniques for solving the above drawback are known as disclosed in Japanese Patent Laid-open Publications 7-288838 and 8-88863. Those techniques are adapted for, while the knee-compression is performed to the largest level of three, R, G, and B, color signals, multiplying the other colors by an common compression rate to maintain the ratio among the three color signals.
However, those techniques fail to perform the compression to the color saturation when the level of input signals increase, although the hue remains intact. A reproduced image may accordingly have a visually unfavorable influence. Also, an image with a higher color saturation saturates even when its brightness is low.
A high-brightness compressing method is provided for maintaining a certain relation among the primary color signals before and after the high-brightness compression process to inhibit a change in hue and to compress the color saturation. Also, a high-brightness compressing circuit is provided for controlling the degree of compression of the color saturation while inhibiting a change in hue.
The high-brightness compressing circuit receives three, R, G, and B, primary color signals and examines their level to determine a maximum color signal, a medium color signal, and a minimum color signal. When any of the three primary color signals exceeds a predetermined level, a compressing circuit performs the compression to them. An output signal corresponding to the medium or minimum color signal is calculated, and then, is released together with the compressed color signals as three primary color signal outputs.